I’m very pleased to announce the merging of ocaml/opam-repository#19855 which restores the ability to compile OCaml 4.12 and earlier on systems with glibc 2.34 (including, but not limited to, Ubuntu 21.10 and Fedora 35).
We have back-ported the fix itself from ocaml/ocaml#10266 (included on 4.13.0/4.13.1) and also ocaml/ocaml#10726 (which will be in 4.13.2/4.14.0) which eliminates the memory leak knowingly introduced by the original fix.
My thanks to @octachron for reviewing the additional patches and @kit-ty-kate for patches to opam-repo-ci to deal with PRs which patch nearly all the compiler packages!
When you need a pause from upgrading all your code to be ready for OCaml 5.00, happy hacking on the older compilers too 